Photographic stencil screen and method of making the same



Get, 5,v 1954 D. SHARPLES PHOTOGRAPHIC STENCIL SCREEN AND METHOD OFMAKING THE SAME Original Filed Oct. 24, 1944 INVENTOR THOMAS, D.SHARPLES,

BY I

ATTORNEY Patented Oct. 5, 1954 UITED STATE OFFICE PHOTOGRAPHIC STENCILSCREEN AND METHOD OF MAKHNG THE SAME Thomas D. Sharples, Schwenksville,Pa., assignor to The Sharples Corporation, a corporation of DelawareClaims.

This invention relates to stencil screens and more particularly tophotographic screens for half-tone printing.

It has long been common to reproduce line subjects photographically bymeans of stencil screens made of textile fabrics. The usual method is tophotograph the line subject and prepare a diapositive from the negativeso obtained. The screen is sensitized by coating with bichromatedgelatin solution and allowed to dry. This is sensitive to light and willbecome insoluble by exposure to light of the violet end of the spectrum.Accordingly, the sensitized screen is exposed to light, behind the linediapositive and, after sufilcient exposure to insure thoroughinsolubilization of the exposed sensitive material the screen is treatedwith hot water. The gelatin not exposed to light, being soluble in hotWater, will dissolve, whereas that exposed to light will not dissolve.It is therefore clear that wherever the diapositive carried a line therewill be a line on the screen, consisting of open pores or meshes in thefabric.

Attempts have heretofore been made to reproduce half-tone subjects bymeans of photographically prepared fabric screen stencils, but seriousdifficulties have been encountered. These prior attempts have involvedthe use of a halftone diapositive, prepared by means of the well knownphotoengravers half-tone screen. One difilculty is that it is necessaryfor the dots of the half-tone screen to be of much coarser grade thanthe mesh of the fabric screen. A second difficulty also arises, namelythat when the two screens are superposed a moire effect is oftenproduced, due to interference between the patterns.

The object of the present invention is to provide a new method forproducing line and halftone screen stencils directly from line orcontinuous tone transparencies without recourse to the half-tone screenor to the special half-tone camera and plate holders of thephotoengraver. With my improved method the difficulties due to moireeffect are entirely eliminated and the resulting half-tone reproductionis of exactly the same grade or texture as the mesh of the fabric screenitself.

A further object of the invention is to provide an improved methodwhereby the above results can be obtained with a minimum of equipmentand with relatively simple processing, so that the practice of theinvention lies well within the abilities and equipment of the usualcommercial photographer.

The present application is a division of my copending application SerialNumber 560,157, filed October 24, 1944, now Patent No. 2,500,877, issuedMarch 14, 1950. While said application covers the complete stencilscreen for half-tone printing, and method of making the same, thepresent application is directed to the prepared textile fabric fromwhich such stencil screens can be produced by photographic processes.

While my improved half-tone stencil screen can be prepared from either apositive or negative transparency, the use of a positive trans parencyor so-called diapositive is somewhat simpler.

In preparing my improved stencil screen, according to one form of theinvention, I employ as a base a piece of woven fabric of suitablefineness made from such materials as cellulose nitrate, regeneratedcellulose (rayon), cellulose acetate, hard gelatin, or other syntheticor natural fibers. The material employed should be such that the fibersthereof tend to absorb water or swell in water or absorb and swell insuch aqueous solutions as it may be found desirable to use in connectionwith this process.

The Warp and Weft strands or filaments of which the textile fabric iswoven have incorporated in them light-sensitive silver salts, such assilver halide salts. These silver salts may be incorporated into thestrands or filaments either during the process of manufacture, or bysubsequent treatment.

The above mentioned woven textile fabric in the strands of which alight-sensitive silver salt is incorporated is then thoroughlyimpregnated with suitable gelatinous material free from silver salts. Asuitable material is medium hard gelatin, relatively insoluble in waterbelow 35 C. and relatively soluble in Water above 45 C.

The sheet of fabric thus treated, after having been dried, is exposed tolight of suitable wave length through a photographic diapositive, eitherby contactor or by projection, so that the silver salts exposed to lightpassing through said diapositive become developable and the portions notexposed to light remain undevelopable. After exposure, the sheet istreated in a suitable developing solution of such a nature that thesilver salts which are more or less exposed to light become more or lessreduced to metallic colloidal silver. After development, the sheet iswashed to remove the developing chemicals and soluble reaction products.

The sheet is then treated with a special hardening and bleachingsolution so that the gelatin becomes more or less insoluble in hotwater, according to the amount of silver present in the fibers orstrands. The sheet is then subjected to the action of hot water in orderto remove all of the water soluble gelatin, while leaving unaffected theportion of gelatin rendered insoluble by the above mentioned bleachingand hardening solution. The screen is then dried and is ready for use.

As a result of the above described process the gelatin surrounding eachstrand of the fabric is rendered insoluble to a greater or lesserdistance from the strand in accordance with the amount of silver presentin that particular strand, so that, after the soluble portions have beenwashed away, there remains a coating of greater or less thickness aroundeach strand. Hence, in certain areas openings will be formed between thecoated strands, these openings being small where the coating is thick,and being larger Where the coating is thinner. In some places, where thegelatin has been completely washed away, the openings are of the fullsize of the mesh of the fabric. There is thus produced a half-tonestencil screen having openings which form the dots in the printedreproduction, such openings lying within the meshes of the fabricbetween the strands thereof and being larger or smaller in accordancewith the depth of tone present in various areas of the originaldiapositive.

In order that the invention may be readily understood, reference is hadto the accompanying drawing forming part of this specification and inwhich:

Fig. l is a fragmentary transverse section through a sheet of textilefabric impregnated and coated in accordance with the invention.

2 is a diagrammatic section through a portion of the finished screenwhich has been most heavily exposed to light, the warp threads beingomitted for the sake of clearness.

Figs. 3, 5 and 7 are fragmentary plan views showing the appearance ofdifferent areas of the finished screen, these areas representing a highlight portion, a middle tone portion and a dark shadow portion of thepicture, respectively, and

Figs. 4, 6 and 8 are views similar to Fig. 2, being sectionssubstantially on the lines il-fl of 3, EiE of Fig. 5, and 8-8 of Fig.'7, respectively, the warp threads being omitted for the sake ofclearness.

Referring to the drawing in detail, my improved screen comprises a pieceof woven textile fabric formed of the usual weft threads or strands i,and the warp threads or strands 2. In these threads have beenincorporated light-sensitive silver salts, and the fabric is impregnatedand coated with gelatin 3, as above described in such manner that thegelatin overlies the silver salts and completely fills the openingsbetween the threads or strands, as shown in Fig. 1.

After this sheet has been exposed to light through a photographictransparency, developed, hardened and subjected to washing in hot water,as described, it will contain some portions, corresponding to theextreme high lights of the picture, in which the gelatin has beenrendered entirely insoluble and hence none of it has been removed by thehot water bath. Such a portion is illustrated in Fig. 2, and it will beseen that the meshes of the fabricv are entirely blocked so that no inkwhatever can pass through, during the printing operation.

In other areas of the screen, corresponding to the medium high lights ofthe picture, some- What less silver has been produced and consequentlycertain portions of the gelatin remained soluble and have been removedby the hot water bath. Thus the gelatin which is insoluble and whichremains in the screen is in the form of a coating around each individualstrand. The depth or thickness of this coating is such that smallopenings or interstices a are formed in the mesh of the fabric betweenthe coated strands, as shown in Figs. 3 and 4.

In other areas of the screen, corresponding to the middle tones of thepicture, still less silver has been produced within the strands, andconsequentl a greater amount of the gelatin remained soluble and hasbeen dissolved by the hot water bath. As a result, the coating remainingaround each strand is thinner, and consequently larger openings b areformed between the coated strands, as shown in Figs. 5 and 6.

In still other areas of the screen, corresponding to the dark tones orshadows of the picture, practically no light has reached the screenduring the exposure through the transparency and consequentlypractically no silver has been produced in the strands of the fabric.Thus none of the gelatin has been rendered insoluble and hencepractically all of it has been washed away in the hot water bath. As aresult there remains nothing but the open mesh of the fabric, as shownin Figs. 7 and 8, with relatively large openings 0 of substantially thefull size of the spaces between the bare strands.

It will therefore be understood that when a screen of this character isemployed as a stencil for printing, the ink will pass through theopenings a, b or 0 onto the printing surface, thus forming small, mediumor large dots in accordance with the lights and shades of the picture,thereby giving the desired half-tone effect.

The weave or mesh of the fabric should not be too close or fine, butshould be more or less open. I have found that from 50 to 89 lines perinch will produce good results, the exact number depending on the natureof the ink and on other conditions. The ink employed should of course beof such consistency that it will not blot or run from one opening toanother, but will produce clear separate dots in at least all portionsof the picture, except perhaps the deepest shadows.

As above stated, there are a number of possible materials that may beused for the strands or filaments from which the textile screen iswoven. An exceptionally suitable material is, however, regeneratedcellulose (rayon).

Methods for forming a silver halide lightsensitive compound inregenerated cellulose are known. One method consists in soaking theregenerated cellulose in a solution of silver nitrate in water and thentreating the material in a suitable soluble halide such as potassiumbromide. Silver bromide will be produced in the regenerated cellulose ina form quite similar to that in which it occurs in ordinary photographicemulsions.

Instead of regenerated cellulose, gelatin photographic emulsion spuninto suitable filaments may be employed. Or, it is possible to usegelatin filaments and form the photographic emulsion within them afterthe textile fabric is woven. The production of silver halide systems ingelatin is well known. Also, methods for producing silver halide systemsin cellulose nitrate are fairly well known and cellulose nitrate maytherefore be also employed.

Another alternative form of material which can be employed is a thin,but highly concentrated photographic emulsion containing silver saltscoated onto the screen fabric after it is woven, care being taken thatthe coating does not plug or block the mesh or openings in the fabric toany appreciable extent. In this case, filaments of non-absorbentmaterial such as glass, or metal, may be employed. After this emulsionis hardened, the screen may be impregnated with the soluble gelation, asabove described.

Still another possible form of material would be filaments having a coreconsisting of fibers, such as sill: or nylon, or even strands of glassor metal, over which is coated a layer of gelatin in which aphotographic emulsion is formed. Such photographic emulsion can beformed in the gelatin either before or after the filaments are woveninto a fabric.

Where, in the appended claims, I use the expression woven textile fabricI intend it to mean a fabric woven from any of the types of strands orfilaments mentioned in the preceding paragraphs.

While I have described the use of gelatin in carrying out the invention,other colloidal material such, for example, as gum arabic, glue, oragar-agar may be employed. I intend to include all of these colloids bythe term gelatinous material.

The fabric sheets above described, incorporating light-sensitive silversalts, and impregnated with soluble gelatinous material, as shown inFig. 1, may then be sold by photographic supply houses as an article ofcommerce, the same as films or sensitized paper, to commercialphotographers or printing establishments desiring to make half-tonestencil screens therefrom, in accordance with the invention.

The following specific example of one exact method of carrying out theinvention is given, assuming that the silver halide used is bromide, asabove described.

Having formed the light-sensitive silver halide system in the filamentsof the woven textile material, in a relatively sensitive yet stableform, the fabric is impregnated with gelatin. This gelatin is made up ina solution of approximately in water, and preservatives againstbacterial decomposition are preferably added. The fabric is then passedthrough this heated solution. The gelatin selected should be of theso-called medium hard or hard variety so that it will withstand thevarious solutions at temperatures around C. without undue swelling orreticulation, or without becoming too delicate to handle readily. Afterthe gelatin is set, it is allowed to dry, in accordance with the usualphotographic practices. The sheet is then ready for use.

The sheet, prepared as above, is then exposed to light under adiapositive of the picture to be reproduced, either by contact or byprojection, or in a camera. The exposing procedure is similar to that ofworking with ordinary photographic printing papers.

The development is carried out in a manner similar to the development ofan ordinary photographic print. A suitable developing solution is asfollows:

Metol (p-methylaminophenol sulfate) gms "1.5 Sodium sulfite gms 45.0

6 Hydroquinone gms 3.0 Sodium carbonate gms 6.0 Potassium bromide gms0.8 Water liter 1.0

If the material has been properly prepared and correctly exposed anddeveloped, the result should look exactly like a correctly exposed anddeveloped paper print.

After washing the developed sheet for about five minutes in runningwater, and drying the same, it is treated in a special hardening andbleaching solution, such, for example, as the following:

Potassium bromide gms 3 Potassium ferricyanide gms 3 Potassiumdichromate gms 3 Chromic acid gms 1 Chrome alum gms 2 Water ml 300 Thissolution penetrates the gelatin rapidly and comes into contact with thesilver compound in the image formed within the filaments of theregenerated cellulose or other textile material. One of the products ofthe resulting bleaching reaction has the property of hardening gelatin.Obviously, the amount of this product formed in the strands or filamentsis directly proportional to the amount of silver present, and thereforeis proportional to the density of the image. Hence, as above described,the thickness of the layer of gelatin surrounding each strand which ishardened depends on the relative density of the silver image in thefabric at each point.

After the bleaching and hardening operation is completed the screen iswashed in hot water. Water at a temperature of approximately 50 C. isabout right, but this will vary somewhat depending on the exactcharacter of the gelatin used, Also, as the screen material gets olderit will be necessary to use a somewhat higher temperature.

As above explained, the effect of this hot water is to wash away thesoluble portions of the gelatin, while the portions which have beenrendered insoluble by the hardening treatment remain on the strands ofthe fabric, with the result that openings of smaller or larger size areformed through the sheet between the coated strands, as above described.

In order to secure additional hardening of the gelatin so as to increasethe useful life of the screen, it may be treated in a bath of 1%formalin made alkaline with 1% of sodium carbonate. After this, thescreen must be washed again for five minutes in running water and maythen be dried. Infra-red heat lamps may be used to secure rapid andsatisfactory drying.

It will, of course, be understood that other known and suitabledevelopers, instead of that above given, may be used if desired. It isalso obvious that other hardening and bleaching mixtures may beemployed, since the properties of the dichromates and other agents forthis purpose are well known in the photographic art.

An example of a suitable alternate developing solution is as follows:

An alternate bleach and hardening process may be used as follows:

The developed, washed and dried sheet is treated in a 2% solution ofpotassium ferricyanide until the visible image is bleached. The sheet isthen placed in a second solution of the following composition:

Potassium bichromate gms 3.0 Potassium bromide gms 5.0 Boric acid gms3.0 Borax gms 3.0 Water ml 300.0

The condition of the sheet is now exactly similar to a sheet treated inthe single solution bleach and hardening solution already described.

If, instead of using a diapositive, it is desired to make a stencilscreen from a photographic negative, the procedure is Slightlydifferent.

In this case, after exposure and development and washing to remove theresulting products, the screen is bleached in a reversal type bleachingsolution that removes the silver that resulted from the development butleaves, virtually unaffected, the silver salts that were not acted on bythe developer.

An example of a suitable bleaching solution that may be used is asfollows:

Water 1iter 1.0 Potassium permanganate gms 3.0 Sulfuric acid, sp. gr.1.87 ml 100 After treatment in this bath, the screen is washed to removethe bleach reaction products.

The remaining silver salts are then fogged so that they becomedevelopable, such foggin being done either by means of light or by meansof suitable known chemical of physical fogging agents. This residualiogged silver salt is then further developed to yield a deposit ofcolloidal silver and the screen is further washed to remove the prodnetsof this second development.

The screen is then subject to the action of a special bleach andhardening solution as before and is treated with hot water, as abovedescribed, to remove all of the soluble gelatin but to leave unaffectedthe gelatin rendered insoluble by the bleachin and hardening treatment.

What I claim is:

1. As an article of manufacture, a photographic stencil screencomprising an open mesh woven textile fabric, in at least the outerportion of the strands of which is incorporated a light-sensitive silversalt which does not appreciably close the mesh openings between thestrands, said fabric being impregnated with soluble gelatinous materialfree from silver salts, which gelatinous material overlies and surroundsthe said silver salt and fills the said mesh openings.

2. As an article of manufacture, a photographic stencil screencomprising an open mesh woven fabric the strands of which are coatedwith an emulsion containing a light sensitive silver salt in such amanner that said coating does not appreciably close the mesh openingsbetween the strands, said fabric being impregnated with solublegelatinous material free from silver salts, which gelatinous materialcompletely fills the mesh openings of the fabric.

8. In the production of a photographic stencil screen, the steps whichcomprise forming a suitable woven fabric'of relatively open mesh,coating said fabric with a photographic emulsion containing alight-sensitive silver salt in such manner that the coating does notappreciably close the mesh openings between the strands, and thenimpregnating the fabric with soluble gelatinous material free fromsilver salts, in such manner that the gelatinous material fills saidmesh openings.

4. In the production of a photographic stencil screen, the steps whichcomprise forming suitable filaments, coating each filament with a layerof material incorporatin a phototgraphic emulsion containing alight-sensitive silver salt, allowing the emulsion to harden, weavingsuch coated filaments into a relatively open mesh fabric, andimpregnating such fabric with soluble gelatinous material free fromsilver salts, in such manner that the gelatinous material completelyfills the mesh openings between the filaments.

References Cited in the file of this patent "UNITED STATES PATENTSNumber Name Date 196,510 Winter Oct. 23, 1877 532,173 Martin Jan. 8,1895 645,952 Harvey Mar. 27, 1900 1,997,745 .Renke Apr. 16, 1935 FOREIGNPATENTS Number Country Date 11,912 Great Britain of 1902 11,219 GreatBritain of 1903 424,976 Great Britain Mar. 6, 1935 446,653 Great BritainPubl. 1936 497,024 Great Britain Dec. 12, 1938 584,530 France Feb. 9,1925

1. AS AN ARTICLE OF MANUFACTURE, A PHOTOGRAPHIC STENCIL SCREENCOMPRISING AN OPEN MESH WOVEN TEXTILE FABRIC, IN AT LEAST THE OUTERPORTION OF THE STRANDS OF WHICH IS INCORPORATED A LIGHT-SENSITIVE SILVERSALT WHICH DOES NOT APPRECIABLY CLOSE THE MESH OPENINGS BETWEEN THESTRANDS, SAID FABRIC BEING IMPREGNATED WITH SOLUBLE GELATINOUS MATERIALFREE FROM SILVER SALTS, WHICH GELATINOUS MATERIAL OVERLIES AND SURROUNDSTHE SAID SILVER SALT AND FILLS THE SAID MESH OPENINGS.